The present invention relates to an oil-impregnated film capacitor or, more particularly, to an oil-impregnated film capacitor of which the dielectric layer consists totally of a film of polypropylene resin, suitable as a circuit element in microwave ovens.
A film capacitor is a device as an important circuit element and has an enrolled structure of a metal (e.g., aluminum) foil to serve as an electrode and a dielectric plastic resin film laid one on the other. Film capacitors are usually impregnated with a capacitor oil to fill the spaces between the aluminum foil and the dielectric film in the enrolled body. In the early stage of development of capacitors of this type, the dielectric layer was provided by a so-called capacitor paper sheet, referred to simply as a paper sheet hereinafter. Since a paper sheet per se is a porous material, it is easy to fully impregnate such an enroller body with a capacitor oil, referred to simply as an oil hereinafter, in addition to the advantages to become little swollen with the oil and to exhibit relatively small thermal expansion and shrinkage by the temperature variations due to use of the capacitor so as to afford good versatility in designing of the capacitors.
On the other hand, paper sheets as a dielectric material in the enrolled body of capacitors have several problems. For example, paper sheets in general have a relatively large dielectric tangent value necessarily leading to large internal heat evolution of the capacitor when on use. In addition, paper sheets generally have a relatively low dielectric breakdown voltage which is inhibitive against increase of the potential gradient between the electrodes necessitating an increase in the thickness of the paper sheet and bulkiness of the capacitor in contrast to the requirement for compactness of various electric and electronic instruments.
Furthermore, a paper sheet as a dielectric material in capacitors is not an advantageous material in respect of resource preservation for paper making and prevention of environmental pollution due to occurrence of a large amount of waste materials including paper debris. These problems give rise to a motivation toward switching of the dielectric sheet material from paper sheets to plastic resin films for cost reduction of capacitor production.
Plastic resin films in general have a relatively high dielectric breakdown voltage enabling a capacitor design with a large potential gradient along with small internal heat evolution and a small capacitor volume per unit dielectric capacity to accomplish compactness of capacitors of a large capacity necessitating a decreased installation area. These advantages are greatly contributing to cost saving in the manufacturing of capacitors as well as in the production cost of instruments using capacitors.
By utilizing the above described advantages of film capacitors, so-called all-film capacitors, in which no dielectric paper sheets are used, are known for power capacitors. Currently, biaxially stretched polypropylene films, referred to simply as OPP films hereinafter, are mainly used as the dielectric sheet material of power capacitors operated at a voltage of 600 V or higher by virtue of their excellent electric properties. A power capacitor of this type usually has a structure as a belt-bundled assembly of several to several tens of unit capacitor elements each in the form of a flattened rolled body prepared by enrolling a dual layer consisting of an OPP film and an aluminum foil. Such a belt-bundled assembly of unit capacitors is impregnated with an oil in a metal casing before hermetic sealing so that the electrostatic capacity of the power capacitor is increased from the value C1 before oil impregnation to a value C2 after oil impregnation. The ratio of this capacity increase C2/C1 is usually in the range from 1.19 to 1.20 in power capacitors, though dependent on various factors.
Different from paper sheets, plastic resin films in general are not porous without micropores and interstices in the internal structure so that, if the plastic resin film is very tightly enrolled to prepare the rolled body, an unduly long time is taken for complete impregnation with the oil. Moreover, plastic resin films generally exhibit larger swelling with the oil than paper sheets along with larger thermal expansion and shrinkage than paper sheets resulting in decreased dimensional stability of the film capacitor.
Due to the above-described characteristics of plastic resin films, various problems and troubles are encountered in the use of an all-film capacitor as a circuit element in microwave ovens. For example, it is a difficult matter to ensure stability of the C2/C1 ratio for the above-mentioned reasons. The large thermal expansion or shrinkage of the resin films as well as swelling of the film with the oil cause deformation or bulging of the casing eventually leading to leakage of the oil. It is sometimes the case that the capacitor casing of an all-film capacitor is bursted. Thus, it is the present status that no success has yet been reached in the development works for an all-film capacitor usable in microwave ovens with good stability of performance.
As an alternative type of the capacitors for microwave ovens, so-called mix-type capacitors are known, in which the technological advantages of paper sheets and plastic resin films for the dielectric layers are utilized in combination, and capacitors of this type are currently under almost exclusive use for microwave ovens.
FIG. 5 of the accompanying drawing is a schematic illustration of the layered structure of a conventional mix-type capacitor used in microwave ovens, which is a rolled body prepared by enrolling a ternary layer consisting of an aluminum foil 4, an OPP film 5 and a paper sheet 6 stacked in this order. The dielectric layer between upper and lower aluminum foils 4,4 is constituted of an OPP film 5 and a single paper sheet or two paper sheets 6. Regardless of the surface condition of the OPP film in the unit capacitor of this mix-type, impregnation with an oil can readily be completed because of the porous nature of the paper sheet or sheets 6. Since almost no changes occur in the form of the paper sheet 6 even by impregnation with an oil, the paper sheet 6 may serve as a cushioning or buffering material against oil-swelling and thermal expansion or shrinkage of the OPP film 5 so as to mitigate the pressure on the rolled body and to avoid undue tightening and deformation of the rolled structure contributing to the stability of the capacitor even without strictly defining the C2/C1 ratio, which is usually in the range from 1.17 to 1.18 when a single paper sheet 6 is used or about 1.20 when two paper sheets 6 are used.